Vertical Distribution Of Suspended Sediment Concentration Research Articles

Effect of the Three Gorges Reservoir on vertical distribution of suspended sediment concentration and estimation accuracy of depth‐average concentration in downstream reaches

AbstractThe sediment regime in the middle Yangtze River has been significantly changed from quasi‐equilibrium to nonequilibrium since the impoundment of the Three Georges Reservoir (TGR). To understand the effects of the TGR on vertical distribution of suspended sediment concentration (SSC) and estimation accuracy of mean concentration, vertical sediment concentration and flow velocity data at the Shashi and Jianli hydrological stations in the reach before and after the impoundment were collected. Comparisons and analysis of vertical profiles of SSC before and after the TGR impoundment show that after the impoundment of the TGR, due to the coarsening of sediment particle size, the reduction in sediment load, and the significant decrease in sediment saturation, the vertical distribution of SSC in the downstream reaches became more uneven under medium and low water flows, which was reflected in the vertical gradient and the fluctuation degree of SSC significantly increased. In addition, the depth‐average sediment concentrations were calculated by the selected‐point method and the mean values calculated by the “multi‐point method” were regarded as the “true mean” to evaluate the accuracy of the mean value calculated by the “few‐point method.” It was found that the relative errors for the selected‐point method were mainly positive before impoundment but mainly negative after impoundment. Additionally, the correction factors of one‐point, two‐point, and three‐point methods and the position of the near‐bed substituted point for the five‐point method were given to reduce the error when the point measurements were used to calculate the depth‐average sediment concentration in the downstream reaches.

Suspended Sediment Concentration Profile in a Typha Latifolia Canopy

AbstractThe transport of sediment in regions with vegetation plays an important role in aquatic ecosystems and landform evolution in river deltas. In this study, flow parameters and suspended sediment concentration (SSC) within an emergent canopy with real plant morphology (Typha latifolia) were measured in a laboratory channel. T. latifolia is a common marsh species with multiple thin leaves emerging from a tight bundle (culm) at the bed. The observed equilibrium SSC profile differed significantly from a Rouse profile, the classic model for SSC over bare beds. The vertical distribution of SSC was nearly uniform in the culm region of the canopy, and decreased with height in the upper canopy region of distributed leaves. The profile of SSC reflected the vertically nonuniform turbulent diffusion, which arose from the plant morphology. A diffusivity model based on cylinder arrays was first validated with tracer data and then adapted to the T. latifolia morphology, providing a way to model turbulent diffusivity in marsh systems. The diffusivity model was successfully used within a random displacement model to predict the spatial evolution and equilibrium SSC profile within the T. latifolia canopy.

Suspended Sediment Flux in the San Francisco Estuary: Part I—Changes in the Vertical Distribution of Suspended Sediment and Bias in Estuarine Sediment Flux Measurements

In this study, we investigate how changes in the vertical distribution of suspended sediment affect continuous suspended sediment flux measurements at a location in the San Francisco Estuary. Current methods for measuring continuous suspended sediment flux estimates relate continuous estimates of suspended-sediment concentration (SSC) measured at-a-point (SSCpt) to discrete cross-section measurements of depth-averaged, velocity-weighted SSC (SSCxs). Regressions that compute SSCxs from continuous estimates of SSCpt require that the slope between SSCpt and SSCxs, controlled by the vertical distribution of SSC, is fixed. However, in tidal systems with suspended cohesive sediment, factors that control the vertical SSC profile—vertical turbulent mixing and downward settling of suspended sediment mediated by flocculation of cohesive sediment—constantly vary through each tide and may exhibit systematic differences between flood and ebb tides (tidal asymmetries in water velocity or particle size). We account for changes in the vertical SSC profile on estimates of SSCxs using time series of the Rouse number of the Rouse-Vanoni-Ippen equation combined with optical turbidity measurements, a surrogate for SSCpt, to predict SSCxs from 2009 to 2011 and 2013. Time series of the Rouse number were estimated by fitting the Rouse-Vanoni-Ippen equation to SSC estimated from optical-turbidity measurements taken at two elevations in the water column. When accounting for changes in the vertical SSC profile, changes in not only the magnitude but also the direction of cumulative sediment-flux measurements were observed. For example, at a mid-depth sensor, sediment flux estimates changed from − 319 kt (± 65 kt, negative indicating net seaward transport) to 482 kt (± 140 kt, positive indicating net landward transport) for 2009–2011 and from − 388 kt (± 140 kt) to 1869 kt (± 406 kt) for 2013–2016. At the study location, estimation of SSCxs solely from SSCpt resulted in sediment flux values that were underestimates on flood tides and overestimates on ebb tides. This asymmetry is driven by covariance between water velocity and particle settling velocity (Ws) with larger Ws on flood compared to ebb tides. Results of this study indicate that suspended-sediment-flux measurements estimated from point estimates of SSC may be biased if systematic changes in the vertical distribution of SSC are unaccounted for.

Vertical Distributions of Suspended Sediment Concentrations in the Turbidity Maximum Zone of the Periodically and Partially Stratified Changjiang Estuary

The vertical distribution of suspended sediment concentration (SSC) plays a key role in the residual suspended sediment transport in the estuarine environments. A field measurement was carried out in the turbidity maximum zone (TMZ) of the Changjiang Estuary in 2012 to study the characteristics and formation mechanisms of SSC profile in the partially stratified estuary. Based on the observed data, we found that the vertical distribution of SSCs is mainly determined by the density stratification caused by the interactions between the river runoff and the saltwater intrusion. The vertical profiles of SSC were categorized into three types: (I) two-layer structure profile, (II) exponential profile, and (III) linear profile. Type I mainly occurred in condition of strong stratification, whereas types II and III mainly occurred in weak stratification and well-mixed conditions, respectively. We developed two new empirical equations to simulate the vertical profiles, which can accurately describe the observed exponential and linear profiles and reasonably/accurately describe the observed two-layer structure profiles. The two new equations can be easily used in other well-mixed or partially stratified estuaries given the surface and bottom SSC data, which can be obtained from many ways, for instance, the buoy and bottom tripod survey system, the remote-sensed images, and numerical models.